Steel material for highly deformable line pipes having superior strain aging resistance and superior HIC resistance, method for manufacturing same, and welded steel pipe

ABSTRACT

A steel material for highly deformable line pipes that has superior strain aging resistance, superior HIC resistance, and a metallographic structure including ferrite, bainite, and martensite-austenite constituent. The area fraction of the martensite-austenite constituent is 0.5 to 5.0%, and the difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness. Additionally, the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less both before a strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment.

TECHNICAL FIELD

The present disclosure relates to a steel material for line pipes that shows less deterioration in material properties after coating treatment at 300° C. or lower, to a method for manufacturing the steel material, and to a welded steel pipe. Specifically, the present disclosure relates to an API X60 to X70 grade steel material for line pipes that has superior HIC resistance in a wet hydrogen sulfide environment with a pH of 5 or more.

BACKGROUND ART

In recent years, there is a need for line pipes used for transporting natural gas and crude oil to have higher strength, in order to improve transport efficiency by high pressure operation. Specifically, the line pipes are required to have high deformability so that the occurrence of cracks can be prevented even when the line pipes are largely deformed by ice gouging or ground deformation. For example, in pipe lines constructed in seismic regions or on the seabed in cold climates where ice gouging occurs, line pipes with high uniform elongation and a low yield ratio of 90% or less are required.

Welded steel pipes such as UOE steel pipes and ERW steel pipes are used for line pipes. Such a welded steel pipe is manufactured by subjecting a steel plate/sheet to cold forming into a pipe shape, welding the seam, and then generally subjecting the outer surface of the steel pipe to coating treatment from the viewpoint of corrosion protection. However, a strain age hardening phenomenon occurs due to work strain imposed during the manufacturing of the pipe and heating during the coating treatment, and this causes an increase in yield stress, so that a problem arises in that the yield ratio of the steel pipe is larger than the yield ratio of the steel plate.

In line pipes used to transport natural gas and crude oil containing hydrogen sulfide, hydrogen generated by the reaction of the hydrogen sulfide and the steel enters the steel, and this may cause cracking. Therefore, such line pipes are required to have hydrogen-induced cracking resistance (HIC resistance) in addition to strength, high uniform elongation, low yield ratio, and strain aging resistance.

One known effective method for achieving a low yield ratio and high uniform elongation is to produce a steel material having a metallographic structure in which hard phases such as bainite and martensite are properly dispersed in a soft phase such as ferrite. One known effective method for preventing hydrogen-induced cracking is to reduce P etc. having a strong tendency to segregation. As gas fields are increasingly developed, a wide variety of sour environment (pH, hydrogen sulfide partial pressure) is being studied, and attention is given to a mildly sour environment (a wet hydrogen sulfide environment). In an environment having a relatively low acidity with a pH of 5 or more, i.e., a so-called mildly sour environment, it is known that addition of Cu to steel to form a protective coating on the steel material is effective in suppressing penetration of hydrogen into the steel.

Patent Literature 1 discloses a manufacturing method for obtaining a structure in which a hard phase is properly dispersed in a soft phase. This manufacturing method includes a heat treatment method in which quenching from a two-phase region of ferrite and austenite is performed between quenching and tempering.

Patent Literature 2 discloses a technique for achieving a low yield ratio without performing the complicated heat treatment disclosed in Patent Literature 1. In this method, rolling of a steel material is finished at a temperature equal to or higher than the Ar₃ temperature, and then the rate of accelerated cooling and cooling stop temperature are controlled to obtain a two-phase structure of acicular ferrite and martensite, whereby a low yield ratio is achieved.

As for the strain aging resistance, Patent Literatures 3 and 4, for example, disclose low-yield ratio, high-strength and high-toughness steel pipes having superior strain aging resistance and methods for manufacturing the steel pipes. Specifically, a fine precipitate of a composite carbide containing Ti and Mo or a fine precipitate of a composite carbide containing at least two of Ti, Nb, and V is utilized.

Patent Literature 5 discloses a method for achieving a low yield ratio, high strength, high uniform elongation, superior strain aging resistance, and API 5L X70 or lower without greatly increasing the amounts of alloy elements added to a steel material. In this method, reheating is performed immediately after accelerated cooling, and a three-phase structure including bainite, polygonal ferrite, and martensite-austenite constituent (MA) is thereby obtained.

Patent Literature 6 discloses a method for achieving HIC resistance in a steel material with X65 or higher and having a two-phase structure of ferrite and bainite. In this method, the difference in hardness between the ferrite and bainite is reduced.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 55-97425

PTL 2: Japanese Unexamined Patent Application Publication No. 1-176027

PTL 3: Japanese Unexamined Patent Application Publication No. 2005-60839

PTL 4: Japanese Unexamined Patent Application Publication No. 2005-60840

PTL 5: Japanese Unexamined Patent Application Publication No. 2011-74443

PTL 6: Japanese Unexamined Patent Application Publication No. 2003-301236

SUMMARY Technical Problem

With the heat treatment method described in Patent Literature 1, a low yield ratio can be achieved by appropriately selecting the quenching temperature from the two-phase region. However, the number of heat treatment steps is large, and this leads to a problem in that productivity decreases and the cost of manufacturing increases.

In the technique described in Patent Literature 2, it is necessary that, in order to obtain a steel material with a tensile strength of 490 N/mm² (50 kg/mm²) or more, the content of carbon in the steel material be increased or a chemical composition including increased amounts of other alloy elements added be used, as shown in Examples in Patent Literature 2. This causes an increase in the cost of raw materials and also results in a problem of deterioration of the toughness of a weld heat affected zone. As described above, a welded steel pipe such as a UOE steel pipe or an ERW steel pipe is produced by subjecting a steel plate/sheet to cold forming into a pipe shape, welding the seam, and then generally subjecting the outer surface of the steel pipe to coating treatment from the viewpoint of corrosion protection etc. Therefore, strain age hardening occurs due to work strain imposed during the manufacturing of the pipe and heating during the coating treatment, and this causes an increase in yield stress. With the technique in Patent Literature 2, although the yield ratio of the raw material steel plate/sheet is reduced, it is difficult to achieve a low yield ratio after the coating treatment.

With the technique described in Patent Literature 3 or 4, the strain aging resistance is improved. However, as shown in Examples in Patent Literatures 3 and 4, no studies were made to ensure the strength of a plate having a large thickness of 26 mm or more. It is difficult to increase the strength of a plate having a large thickness of 26 mm or more because of a reduction in cooling rate due to the large thickness. A multi-specification material of API 5L X65 to X70 having a large thickness, high deformability, strain aging resistance, and mild-sour resistance has not been developed.

With the technique described in Patent Literature 5, a low yield ratio of 85% or less is achieved after strain aging treatment, as shown in Examples in Patent Literature 5. However, it is feared that hydrogen-induced cracking will occur in a wet hydrogen sulfide environment.

With the technique described in Patent Literature 6, high HIC resistance is achieved in a wet hydrogen sulfide environment with a pH of 3.3 or higher. However, since it is necessary to reduce the difference in hardness between the ferrite and bainite, a low yield ratio may not be achieved. Material design for steel used in a severe sour environment, such as high cleanliness of steel components, is excessive for welded steel pipes used under a mildly sour environment and causes a problem of an increase in manufacturing cost.

It is an object of the present disclosure to provide an API 5L X60 to X70 grade steel material for highly deformable line pipes that exhibits superior HIC resistance in a wet hydrogen sulfide environment with a pH of 5 or more and has a low yield ratio even after coating treatment and to provide a method for manufacturing the steel material and a welded steel pipe.

Solution to Problem

To achieve the foregoing object, the present inventors have conducted extensive studies on an appropriate chemical composition and a method for manufacturing a steel material, particularly on a manufacturing process including controlled rolling and accelerated cooling after the controlled rolling and have found the following.

(a) The HIC resistance can be improved by adding an appropriate amount of Cu while no Mo is contained or, even when Mo is contained, its content is 0.01% or less.

(b) Cooling start temperature in an accelerated cooling process is controlled appropriately, and the cooling is stopped during bainite transformation, i.e., in a temperature range in which untransformed austenite is present. Then the steel plate is reheated from a temperature equal to or higher than bainite transformation finish temperature (hereinafter referred to as “Bf point”). In this case, the metallographic structure of the steel plate is a three-phase structure in which martensite-austenite constituent (hereinafter denoted as MA), which is a hard phase, is uniformly formed in a mixed phase of ferrite and bainite. This allows a low yield ratio to be achieved both before strain aging treatment and after the strain aging treatment (hereinafter may be referred to as both before and after the strain aging treatment).

(c) By setting the cooling start temperature and cooling stop temperature during the accelerated cooling to appropriate temperatures, the amount of solute C can be reduced, so that an increase in the yield ratio after strain aging can be suppressed.

The disclosed embodiments have been made on the basis of the above findings and further studies and is as follows.

[1] A steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance, the steel material having a chemical composition comprising, in mass %, C: 0.030 to 0.100%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.5%, P: 0.015% or less, S: 0.002% or less, Cu: 0.20 to 1.00%, Mo: 0.01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.040%, Al: 0.10% or less, and N: 0.007% or less, with the balance being Fe and inevitable impurities, wherein the steel material has a metallographic structure including ferrite, bainite, and martensite-austenite constituent, wherein the area fraction of the martensite-austenite constituent is 0.5 to 5.0%, wherein the difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness, and wherein, both before strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less.

[2] The steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance according to [1], wherein the chemical composition further comprises, in mass %, one or at least two of Ni: 0.02 to 0.50%, Cr: 1.00% or less, V: 0.10% or less, Ca: 0.0050% or less, and B: 0.0050% or less.

[3] A method for manufacturing a steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance, the method comprising: heating steel having the chemical composition according to [1] or [2] to a temperature of 1,000 to 1,300° C.; subjecting the resulting steel to hot rolling at a finish rolling temperature equal to or higher than Ar₃ temperature; then subjecting the resulting steel to accelerated cooling from a cooling start temperature of (Ar₃−50) to (Ar₃+30)° C. to a cooling stop temperature of 450 to 650° C. at a cooling rate of 5° C./s or more; and reheating the resulting steel to 550° C. to 750° C. at a heating rate of 0.5° C./s or more immediately after the accelerated cooling; wherein the steel material has a metallographic structure including ferrite, bainite, and martensite-austenite constituent, wherein the area fraction of the martensite-austenite constituent is 0.5 to 5.0%, wherein the difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness, and wherein, both before strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less.

[4] A welded steel pipe produced using the steel material according to [1] or [2] as a raw material.

Advantageous Effects

According to the present disclosure, an API 5L X60 to X70 grade steel material for highly deformable line pipes can be obtained, which exhibits superior HIC resistance in a wet hydrogen sulfide environment with a pH of 5 or more and has a low yield ratio even after coating treatment at 300° C. or lower.

The strain aging resistance in the present disclosure is a property that allows an excessive increase in yield ratio to be suppressed even when heat treatment at a temperature of 300° C. or lower is performed. The HIC resistance in the present disclosure is such a property that hydrogen-induced cracking is prevented from occurring in a wet hydrogen sulfide environment with a pH of 5 or more. The high deformability is such a property that the uniform elongation is 9% or more and the yield ratio is 90% or less.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The disclosed exemplary embodiments will next be specifically described.

1. Chemical Composition

A description will next be given of the reasons for the limitations on the chemical composition of the steel material according to the disclosed embodiments. The unit “%” for each component means % by mass.

C: 0.030 to 0.100%

C is an element that contributes to precipitation strengthening in the form of carbide. If C is less than 0.030%, the formation of MA (martensite-austenite constituent) is insufficient. In this case, sufficient strength cannot be ensured, and the difference in hardness between the ferrite and bainite cannot be ensured to be a prescribed value or more, so that the yield ratio becomes large. If C exceeds 0.100%, toughness and weldability deteriorate, and strain aging causes an increase in the yield ratio. Therefore, the content of C is specified to be 0.030 to 0.100%. Preferably, the content of C is 0.05% or more. Preferably, the content of C is 0.09% or less.

Si: 0.01 to 0.50%

Si is contained for the purpose of deoxidization. If Si is less than 0.01%, the deoxidization effect is insufficient. If Si exceeds 0.50%, the toughness and weldability deteriorate. Therefore, the content of Si is specified to be 0.01 to 0.50%. Preferably, the content of Si is 0.01 to 0.3%.

Mn: 0.5 to 2.5%

Mn is contained for the purpose of strength and toughness. If Mn is less than 0.5%, its effect is insufficient. In addition, the amount of MA (martensite-austenite constituent) is insufficient, so that the yield ratio becomes large. Therefore, the content of Mn is 0.5% or more. From the viewpoint of achieving a low yield ratio through the formation of MA, the lower content of Mn is preferably 1.2% or more and more preferably 1.5% or more. If Mn exceeds 2.5%, the toughness and weldability deteriorate. Therefore, the upper content of Mn is specified to be 2.5% or less and is preferably 2.0% or less.

P: 0.015% or Less

P is an inevitable impurity element that causes deterioration of the weldability and HIC resistance. Therefore, the content of P is specified to be 0.015% or less. Preferably, the content of P is 0.010% or less.

S: 0.002% or Less

Generally, S forms MnS inclusions in the steel, and this causes deterioration of the HIC resistance. Therefore, the content of S is preferably as small as possible. When S is 0.002% or less, S causes no problems. Therefore, the upper limit of the content of S is specified to be 0.002%. Preferably, the content of S is 0.0015% or less.

Cu: 0.20 to 1.00%

Cu is an important element in the present disclosure. Cu suppresses penetration of hydrogen into the steel and contributes to an improvement of the HIC resistance. However, if Cu is less than 0.20%, its effect is insufficient. If Cu exceeds 1.00%, the weldability deteriorates. Therefore, the content of Cu is specified to be 0.20 to 1.00%. Preferably, the content of Cu is 0.25% or more. Preferably, the content of Cu is 0.5% or less.

Mo: 0.01% or Less (Including 0)

Mo causes an increase in yield ratio by strain aging and deterioration of the HIC resistance. Therefore, no Mo is contained, or, even when Mo is contained, the content of Mo is specified to be 0.01% or less. Preferably, the content of Mo is 0.005% or less.

Nb: 0.005 to 0.05%

Nb improves the toughness through refinement of the structure and forms carbide to thereby contribute to an increase in strength. However, if Nb is less than 0.005%, its effect is insufficient. If Nb exceeds 0.05%, the toughness of a weld heat affected zone deteriorates. Therefore, the content of Nb is specified to be 0.005 to 0.05%. Preferably, the content of Nb is 0.01 to 0.05%.

Ti: 0.005 to 0.040%

Ti suppresses coarsening of austenite during heating of a slab through the pinning effect of TiN, improves the toughness of the base material, reduces the amount of solute N, and suppresses an increase in yield ratio by strain aging. However, if Ti is less than 0.005%, its effect is insufficient. If Ti exceeds 0.040%, the toughness of a weld heat affected zone deteriorates. Therefore, the content of Ti is specified to be 0.005 to 0.040%. Preferably, the content of Ti is 0.005 to 0.02%.

Al: 0.10% or Less

Al is contained as a deoxidizer. If Al exceeds 0.10%, the cleanliness of the steel is reduced, and the toughness deteriorates. Therefore, the content of Al is specified to be 0.10% or less. Preferably, the content of Al is 0.01 to 0.08%.

N: 0.007% or Less

N is an inevitable impurity element that causes an increase in yield ratio by strain aging and deterioration of the toughness of a weld heat affected zone. Therefore, the upper limit of the content of N is specified to be 0.007%. Preferably, the content of N is 0.006% or less.

The above-described components are basic components of the present disclosure. For the purpose of further improving the strength and toughness of the steel plate and also increasing its HIC resistance, one or at least two of Ni, Cr, V, Ca, and B may be contained.

Ni: 0.02 to 0.50%

Ni is an element that contributes to an improvement of the HIC resistance and is effective in improving the toughness and increasing the strength. If Ni is less than 0.02%, its effect is insufficient. If the content of Ni exceeds 0.50%, its effect is saturated, and this is disadvantageous in terms of cost. Therefore, when Ni is contained, the content of Ni is specified to be 0.02 to 0.50%. Preferably, the content of Ni is 0.2% or more. Preferably, the content of Ni is 0.4% or less.

Cr: 1.00% or Less

Cr is an element effective in obtaining sufficient strength even at low C content. If Cr exceeds 1.00%, the weldability deteriorates. Therefore, when Cr is contained, the upper limit of the content of Cr is specified to be 1.00%. Preferably, the content of Cr is 0.1 to 0.5%.

V: 0.10% or Less

V improves the toughness through refinement of the structure and forms carbide to thereby contribute to an increase in strength. If V exceeds 0.10%, the toughness of a weld heat affected zone deteriorates. Therefore, when V is contained, the content of V is specified to be 0.10% or less. Preferably, the content of V is 0.005% or more. Preferably, the content of V is 0.05% or less.

Ca: 0.0050% or Less

Ca is an element effective in improving the toughness through control of the form of sulfide-based inclusions. If Ca exceeds 0.0050%, its effect is saturated, and the toughness rather deteriorates because of a reduction in the cleanliness of the steel. Therefore, when Ca is contained, the content of Ca is specified to be 0.0050% or less. Preferably, the content of Ca is 0.001% or more. Preferably, the content of Ca is 0.004% or less.

B: 0.0050% or Less

B is an element effective in increasing the strength and improving the toughness of a weld heat affected zone. If B exceeds 0.0050%, the weldability deteriorates. Therefore, when B is contained, the content of B is specified to be 0.0050% or less. Preferably, the content of B is 0.003% or less and 0.0003% or more.

In the steel material of the disclosed embodiments, the balance other than the above-described components is Fe and inevitable impurities. However, elements other than above-described elements may be contained without any problems, so long as the operational advantages of the present disclosure are not impaired.

2. Metallographic Structure

The metallographic structure of the steel plate of the present disclosure is composed mainly of a three-phase structure including ferrite, bainite, and martensite-austenite constituent. The structure composed mainly of the three-phase structure including ferrite, bainite, and martensite-austenite constituent is a multi-phase structure in which the total area fraction of the ferrite, the bainite, and the martensite-austenite constituent is 90% or more. The remainder is a structure including one or at least two selected from martensite (except for the martensite-austenite constituent), pearlite, retained austenite, etc. and having a total area fraction of 10% or less.

The area fraction of the martensite-austenite constituent is 0.5 to 5.0%. This allows the yield ratio to be 90% or less both before and after the strain age hardening treatment. An area fraction of the martensite-austenite constituent of less than 0.5% may be insufficient for achieving a low yield ratio. If the area fraction of the martensite-austenite constituent exceeds 5.0%, the toughness of the base material and the HIC resistance may deteriorate. The martensite-austenite constituent is not decomposed at heating temperature (up to 300° C.) during coating treatment and is stable. Therefore, in the present disclosure, a low yield ratio can be achieved even after the coating treatment. During the heat treatment in the coating treatment, the strain age hardening phenomenon occurs. Therefore, when a low yield ratio is achieved both before the strain aging treatment and after the strain aging treatment, a low yield ratio can be achieved even after the coating treatment performed when a welded steel pipe is manufactured. It is not particularly necessary to limit the area fractions of the ferrite and the bainite. However, from the viewpoint of achieving a low yield ratio and HIC resistance, the area fraction of the ferrite is 10% or more, and the area fraction of the bainite is 10% or more.

The difference in hardness between the ferrite and bainite is 60 or more in terms of Vickers hardness (HV). When the difference in hardness is 60 or more, the yield ratio can be 90% or less both before and after the strain age hardening treatment. A difference in hardness of less than 60 HV results in the same behavior as that of a ferrite or bainite single phase structure, and the yield ratio becomes high, so that it is difficult to achieve the desired yield ratio. If the difference in hardness is more than 180 HV, the HIC resistance may deteriorate, and the yield ratio after the strain aging may increase. Therefore, the difference in hardness is preferably 180 HV or less. The difference in hardness is more preferably 150 HV or less.

The types of the metallographic structures and the area fraction of each phase can be determined by observation under, for example, an optical microscope or a scanning electron microscope. Specifically, microstructure photographs of at least three regions are taken and subjected to image processing. After etching with, for example, a 3% nital solution (nital: a nitric acid-alcohol solution) and then electrolytic etching, the MA (martensite-austenite constituent) can be easily identified by observation. The area fraction of the MA can be determined by observation under a scanning electron microscope. Specifically, microstructure photographs of at least three regions are taken and subjected to image processing.

The hardness is a value measured using a Vickers hardness tester, and any load can be selected so that an indentation with an optimal size is obtained inside each phase. Preferably, the same load is used to measure the hardness of the ferrite and the hardness of the bainite. In consideration of local variations in the composition of the microstructure and variations due to measurement errors, it is preferable that the hardness measurement is performed at at least 15 different positions for each of the phases and that the average hardnesses of these phases are used as the hardnesses of the ferrite and bainite. The difference in hardness when the average hardnesses are used is the absolute value of the difference between the average hardness of the ferrite and the average hardness of the bainite.

3. Tensile Properties Before and after Strain Aging Treatment

Both before strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the uniform elongation is 9% or more, and the yield ratio is 90% or less

A steel material for line pipes used in seismic regions is required to be highly deformable so that no fracture occurs even under large deformation such as ground deformation. In addition, it is necessary that the high deformability be maintained even after the strain aging treatment in which the steel material is heated up to 300° C. for coating the steel material for corrosion protection. When, both before the strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the uniform elongation is 9% or more and the yield ratio is 90% or less, sufficiently high deformability is obtained, so that fracture may not occur due to large deformation such as an earthquake. Therefore, in the steel material of the disclosed embodiments, both before the strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the uniform elongation is 9% or more, and the yield ratio is 90% or less. It is preferable that, both before the strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment, the uniform elongation is 10% or more and the yield ratio is 88% or less, from the viewpoint of high deformability.

4. Manufacturing Conditions

Next, a method for manufacturing the steel material for highly deformable line pipes according to the present disclosure will be described. The method for manufacturing the steel material for highly deformable line pipes according to the present disclosure includes: subjecting a steel raw material having the above-described chemical composition to hot rolling at a heating temperature of 1,000 to 1,300° C. and a finish rolling temperature of Ar₃ temperature or higher; subjecting the resulting steel material to accelerated cooling from a cooling start temperature of (Ar₃−50) to (Ar₃+30)° C. to a cooling stop temperature of 450 to 650° C. at a cooling rate of 5° C./s or more; and reheating the resulting steel to 550 to 750° C. at a heating rate of 0.5° C./s or more immediately after the accelerated cooling; whereby the desired metallographic structure can be obtained. The temperature is the temperature at a central portion of the steel material. The Ar₃ temperature is calculated from the following formula. Ar₃ (° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo

In the above formula, each atomic symbol represents the content (mass %) of the element and is 0 when the element is not contained.

Next, a description will be given of the reasons for the limitations on the conditions of manufacturing.

Heating Temperature: 1,000 to 1,300° C.

If the heating temperature is lower than 1,000° C., the dissolution of carbides is insufficient, so that the required strength is not obtained. If the heating temperature exceeds 1,300° C., the toughness of the base material deteriorates. Therefore, the heating temperature is specified to be 1,000 to 1,300° C.

Finish Rolling Temperature: Ar₃ Temperature or Higher

If the finish rolling temperature is lower than the Ar₃ temperature, the rate of ferrite transformation after the rolling is reduced, and plastic strain caused by the rolling remains in the ferrite. In this case, the strength of the ferrite becomes high, and the difference in hardness between the ferrite and bainite becomes low, so that the desired yield ratio cannot be achieved. Therefore, the finish rolling temperature is specified to be equal to or higher than the Ar₃ temperature. Preferably, a cumulative rolling reduction ratio in a temperature range of 900° C. or lower is 50% or more. When the cumulative rolling reduction ratio in the temperature range of 900° C. or lower is 50% or more, the size of austenite grains can be reduced.

Cooling Start Temperature in Accelerated Cooling: (Ar₃−50) to (Ar₃+30)° C.

If the cooling start temperature is lower than (Ar₃−50)° C., the area fraction of the ferrite increases, and the strength of the base material is reduced. In addition, the difference in hardness between the ferrite and bainite becomes large, and the HIC resistance deteriorates. Therefore, the cooling start temperature is (Ar₃−50)° C. or higher and preferably (Ar₃−30)° C. or higher. If the cooling start temperature exceeds (Ar₃+30)° C., the area fraction of the ferrite decreases, and the difference in hardness between the ferrite and bainite decreases and is insufficient for achieving a low yield ratio. Therefore, the cooling start temperature is (Ar₃+30)° C. or lower and preferably (Ar₃+25)° C. or lower.

Cooling Rate in Accelerated Cooling: 5° C./s or More

If the cooling rate is less than 5° C./s, pearlite is formed during cooling, so that sufficient strength and a sufficiently low yield ratio are not obtained. Therefore, the cooling rate is specified to be 5° C./s or more. The cooling rate is preferably 8° C./s or more and more preferably 10° C./s or more. The cooling rate is preferably 100° C./s or less and more preferably 60° C./s or less.

Cooling Stop Temperature: 450 to 650° C.

In the present disclosure, the cooling stop temperature in the accelerated cooling is an important manufacturing condition. In the present disclosure, untransformed austenite which is present after reheating and in which C is concentrated is transformed to MA (martensite-austenite constituent) during air cooling after the reheating. Specifically, it is necessary to stop the cooling during bainite transformation within a temperature range in which untransformed austenite is present. If the cooling stop temperature is lower than 450° C., the bainite transformation is completed, and therefore MA (martensite-austenite constituent) is not formed during air cooling, so that a low yield ratio cannot be achieved. If the cooling stop temperature is higher than 650° C., C is consumed by pearlite precipitated during cooling. In this case, the formation of MA (martensite-austenite constituent) is suppressed, and the amount of MA is insufficient. Therefore, the cooling stop temperature in the accelerated cooling is specified to be 450 to 650° C. The cooling stop temperature is preferably 515° C. or higher and more preferably 530° C. or higher. The cooling stop temperature is preferably 635° C. or lower and more preferably 620° C. or lower.

In the disclosed embodiments, it is preferable to perform the reheating immediately after the accelerated cooling. This is because it is preferable to start the reheating from a state in which untransformed austenite is present. The term “immediately” is preferably within 120 seconds after the cooling is stopped, from the viewpoint of manufacturing efficiency and a reduction in the cost of fuel required for the heat treatment.

Reheating to a temperature of 550 to 750° C. at a heating rate of 0.5° C./s or more immediately after the accelerated cooling is stopped

This process is also an important manufacturing condition in the present disclosure. During the reheating, the untransformed austenite undergoes ferrite transformation, and C is thereby evacuated to the untransformed austenite. The untransformed austenite in which C is concentrated is transformed into MA (martensite-austenite constituent) during air cooling after the reheating. To obtain the MA (martensite-austenite constituent), it is necessary to perform the reheating from a temperature equal to or higher than the Bf point to a temperature range of from 550 to 750° C. after the accelerated cooling. If the heating rate is less than 0.5° C./s, the time until the target reheating temperature is reached becomes long, so that the manufacturing efficiency deteriorates. In addition, since pearlite transformation occurs, MA (martensite-austenite constituent) is not obtained, and a sufficiently low yield ratio cannot be obtained.

If the reheating temperature is lower than 550° C., the ferrite transformation does not occur sufficiently, so that C is not evacuated to the untransformed austenite sufficiently. In this case, MA is not formed, and a low yield ratio cannot be achieved. If the reheating temperature exceeds 750° C., the bainite softens. In this case, the difference in hardness between the ferrite and bainite becomes less than 60 HV, and a low yield ratio cannot be achieved. Therefore, the reheating temperature range is specified to be 550 to 750° C. For the ferrite transformation to occur reliably to thereby concentrate C in the untransformed austenite, it is preferable that the reheating is performed to a temperature higher than the reheating start temperature by at least 50° C. Preferably, the rate of cooling after the reheating is basically equal to the rate of air cooling.

Through the manufacturing process described above, a steel material for highly deformable line pipes can be obtained, which exhibits superior strain aging resistance and HIC resistance and has a uniform elongation of 9% or more and a yield ratio of 90% or less both before and after the strain aging treatment at a temperature of 300° C. or lower. In the present disclosure, even after the steel material is subjected to a thermal history at a temperature of 300° C. or lower in a general coating process for steel pipes, an increase in yield ratio and a reduction in uniform elongation that are caused by strain aging can be suppressed, and a uniform elongation of 9% or more and a yield ratio of 90% or less can be ensured. A strain age hardening phenomenon occurs during the heat treatment in the coating treatment. Therefore, by achieving a low yield ratio both before the strain aging treatment and after the strain aging treatment, a low yield ratio can be achieved even when the coating treatment is performed during manufacturing of a welded steel pipe.

5. Method for Manufacturing Welded Steel Pipe

A method for manufacturing a welded steel pipe will next be described.

In the disclosed embodiments, the above-descried steel material is used to form a steel pipe. In one method for forming a steel pipe, the steel material is formed into a steel pipe shape through cold forming such as a UOE process or press bending (referred to also as bending press).

In the UOE process, lateral edges of a steel plate used as a raw material are beveled and then crimped using a press machine. Then the steel plate is formed into a U shape and then into an O shape using a press machine. In this manner, the steel plate is formed into a cylindrical shape with the lateral edges of the steel plate facing each other. Then the lateral edges of the steel plate are brought into abutment and welded. This welding is referred to as seam welding. Preferably, the seam welding is performed using a method including two steps, i.e.: a tack welding step of holding the steel plate having the cylindrical shape, bringing the facing lateral edges of the steel plate into abutment, and performing tack welding; and a final welding step of subjecting the inner and outer surfaces of the seam of the steel plate to welding using a submerged arc welding method. After the seam welding, pipe expansion is performed in order to remove welding residual stress and to improve the roundness of the steel pipe. In the pipe expansion step performed, the pipe expansion ratio (the ratio of a change in outer diameter of the pipe before and after the pipe expansion to the outer diameter of the pipe before the pipe expansion) is usually 0.3% to 1.5%. From the viewpoint of the balance between the roundness improvement effect and the required ability of the pipe expansion machine, the pipe expansion ratio is preferably within the range of 0.5% to 1.2%. Subsequently, coating treatment may be performed for the purpose of corrosion protection. In the coating treatment, the outer surface may be heated to a temperature range of, for example, 200 to 300° C. and then coated with a known resin.

In the press bending, a steel plate is repeatedly subjected to three-point bending to gradually change its shape to thereby manufacture a steel pipe having a substantially circular cross section. Then seam welding is performed, as in the UOE process described above. Also in the press bending, pipe expansion may be performed after the seam welding, and a coating may also be formed.

Example 1

Steel (one of steel types A to K) having a chemical composition shown in Table 1 (the balance being Fe and inevitable impurities) was used to manufacture a steel material having a plate thickness of 30 mm or 33 mm under conditions shown in Table 2. The reheating was performed using an induction heating furnace or a gas-fired furnace. The temperature of a central portion of the steel plate was used as temperatures such as the heating temperature, finish rolling temperature, cooling stop (finish) temperature, and reheating temperature. The temperature of the central portion was measured directly by inserting a thermocouple into the central portion of a slab or the steel plate or was calculated from the surface temperature of the slab or the steel plate using parameters such as the plate thickness and the thermal conductivity. The cooling rate is the average cooling rate calculated by dividing the temperature difference required for cooling to the cooling stop (finish) temperature after completion of the hot rolling by the time required for the cooling. The reheating rate (heating rate) is the average heating rate calculated by dividing the temperature difference required for reheating to the reheating temperature after the cooling by the time required for the reheating.

TABLE 1 Ar₃ Steel Chemical composition (mass %) temperature type C Si Mn P S Cu Ni Cr Mo Nb V Ti Al Ca B N (° C.) Remarks A 0.053 0.12 1.9 0.008 0.001 0.22 — — — 0.04 0.001 0.010 0.03 0.0028 — 0.003 737 Within the B 0.076 0.10 1.8 0.005 0.001 0.28 0.28 — — 0.02 — 0.010 0.03 0.0027 — 0.004 721 scope of C 0.076 0.10 1.5 0.006 0.001 0.30 0.28 — — 0.02 — 0.010 0.03 0.0023 — 0.003 745 the present D 0.075 0.11 1.8 0.006 0.001 0.28 0.28 0.30 — 0.02 — 0.011 0.05 0.0031 — 0.003 717 disclosure E 0.085 0.06 1.5 0.008 0.001 0.30 0.30 — — 0.03 0.04 0.013 0.03 0.0025 0.0010 0.004 741 F 0.083 0.31 0.8 0.009 0.002 0.43 0.45 — 0.01 0.03 — 0.012 0.03 — — 0.004 786 G 0.071 0.16 1.8 0.005 0.001 0.28 0.27 — 0.07 0.02 — 0.013 0.03 0.0015 — 0.003 718 Outside the H 0.023 0.38 2.4 0.008 0.002 0.20 0.22 — — 0.03 0.04 0.010 0.03 — — 0.005 695 scope of I 0.094 0.34 0.4 0.009 0.001 0.51 0.46 — — 0.03 — 0.012 0.03 — — 0.004 813 the present J 0.085 0.24 2.1 0.012 0.002 0.07 0.30 — — 0.03 — 0.011 0.03 — — 0.004 698 disclosure K 0.124 0.22 1.7 0.008 0.001 0.21 0.21 — — 0.04 — 0.010 0.03 — 0.0007 0.004 720 *Underlined items are outside the scope of the present disclosure.

TABLE 2 Cooling Cooling Plate Heating Finish rolling start Cooling stop Reheating Reheating Steel thickness temperature temperature temperature rate temperature Reheating rate temperature No. type (mm) (° C.) (° C.) (° C.) ( °C./s) (° C.) facility ( °C./s) (° C.) Remarks 1 A 33 1100 820 750 20 590 Induction 5   650 Within the heating furnace scope of 2 B 30 1050 760 730 20 570 Induction 8   670 the present heating furnace disclosure 3 B 30 1150 730 680 40 550 Induction 5   650 heating furnace 4 C 30 1020 750 730 25 520 Induction 3   660 heating furnace 5 D 33 1050 760 740 20 620 Gas-fired 1   660 furnace 6 E 33 1280 750 720 30 520 Induction 5   680 heating furnace 7 F 30 1150 810 772 40 560 Induction 6   670 heating furnace 8 A 30 1100 820 760 20 660 Induction 5   680 heating furnace Outside the 9 B 30 1100 790 750  3 580 Induction 3   650 scope of heating furnace the present 10 B 30  950 770 740 25 650 Induction 7   660 disclosure heating furnace 11 B 30 1100 780 755 25 580 Induction 7   660 heating furnace 12 C 30 1120 780 730 30 550 Induction 0.3 670 heating furnace 13 C 33 1080 780 740 40 510 Induction 7   540 heating furnace 14 G 33 1150 780 730 25 560 Induction 7   670 heating furnace 15 H 30 1180 750 720 40 600 Induction 7   650 heating furnace 16 I 30 1250 840 820 35 550 Induction 8   670 heating furnace 17 J 30 1100 770 647 40 540 Induction 5   660 heating furnace 18 K 30 1180 780 730 35 570 Induction 3   650 heating furnace *Underlined items are of the outside the scope present disclosure.

For each of the steel materials manufactured as described above, structure observation was performed, and the tensile properties, the difference in hardness, and the HIC resistance were measured. The methods for evaluation are as follows.

(1) Structure Observation

A test piece for the structure observation was taken from one of the steel plates obtained, and an L direction cross section was polished and etched with nital. Microstructures of at least three regions in a central portion in a thickness direction, i.e., a portion ±2 mm from the center position in the thickness direction, were observed under an optical microscope (magnification: 400×) or a scanning electron microscope (magnification: 2,000×). The images of the microstructures were taken and subjected to image analysis to determine the type of the structure and the area fraction of each phase.

(2) Tensile Properties

To evaluate the tensile strength before strain aging treatment, two No. 4 test pieces specified in JIS Z 2201 were taken in a direction perpendicular to the rolling direction and subjected to a tensile test, and the average of the test values was used for evaluation. The strength required in the present disclosure is a tensile strength of 517 MPa or more (API 5L X60 or more). To evaluate the yield ratio and uniform elongation, two No. 4 test pieces specified in JIS Z 2201 were taken in the rolling direction and subjected to a tensile test, and the averages of the test values were used for evaluation. The yield ratio required in the present disclosure is a yield ratio of 90% or less, and the uniform elongation is 9% or more.

To evaluate the tensile strength after strain aging treatment, two No. 4 test pieces specified in JIS Z 2201 were taken in the rolling direction, subjected to a tensile strain of 2.0%, and then held at 250° C. for 5 minutes to perform strain aging treatment. Then a tensile test was performed, and the tensile strength was evaluated using the average of the test values. The evaluation criteria after the strain aging treatment are the same as the above-described evaluation criteria before the strain aging treatment.

(3) Difference in Hardness

A test piece for hardness measurement was taken from one of the steel plates obtained. The hardness of the ferrite and the hardness of the bainite were measured using a Vickers hardness tester with a measurement load of 5 g, and averages of at least 10 measurements were used to determine the difference in hardness between the ferrite and bainite.

(4) HIC Resistance

As for the HIC Resistance, an HIC Test was Performed under the following conditions: immersion for 96 hours in a 1 mol/L acetic acid buffer solution completely saturated with hydrogen sulfide, having a pH of about 5.0, and containing 5% NaCl. When no cracking was found, the HIC resistance was judged as good and represented by “Good.” When cracking was found, a “Poor” rating was assigned.

The measurement results are shown in Table 3. In all the metallographic structures except for the one specified, the area fraction of the ferrite was 10% or more, and the area fraction of the bainite was 10% or more (in No. 11, the area fraction of the ferrite was 2%).

TABLE 3 Metallographic structure Total area Difference fraction of Area in hardness Before aging After aging ferrite, fraction between Tensile Yield Uniform Tensile Yield Uniform Steel bainite, and of ferrite and strength ratio elongation strength ratio elongation HIC No. type MA (%) Remainder MA (%) bainite (HV) (MPa) (%) (%) (MPa) (%) (%) resistance Remarks 1 A 100 — 4.8  72 565 87 16 576 86 12 Good Inventive 2 B 99 P 2.2  68 548 88 19 553 88 15 Good Examples 3 B 96 P 2.1 152 535 83 14 540 89 12 Good 4 C 100 — 0.8  65 525 86 17 541 85 13 Good 5 D 99 P 1.7  74 600 84 16 598 85 13 Good 6 E 100 — 4.5  80 593 85 14 591 86 11 Good 7 F 100 — 0.5  87 521 89 17 537 90 14 Good 8 A 98 P 0.4  50 529 91 10 532 91  7 Good Comparative 9 B 85 P 0.3  61 531 92 13 537 92 10 Good Examples 10 B 98 P 0.9  58 505 91 9 516 92  7 Good 11 B 100 — 0.6  35 590 92  8 597 93  8 Good (ferrite: 2%) 12 C 96 P 0.2  43 545 92 10 552 92  8 Good 13 C 100 — 0.1  66 610 91 9 596 92  7 Good 14 G 99 P 4.7  75 612 75 16 611 82 12 Poor 15 H 100 — 0.1  55 591 90  8 595 91  7 Good 16 I 100 — 0.2  62 487 92 16 490 92 15 Good 17 J 100 — 1.9 185 610 84 13 614 90 12 Poor 18 K 100 — 8.5  85 652 77 9 638 91  7 Good *Underlined items are outside the scope of the present disclosure. MA: Martensite-austenite constituent, P: Perlite

As shown in Table 3, in each of Nos. 1 to 7, which are Inventive Examples, the chemical composition and the manufacturing method were within the scope of the present disclosure. Both before and after the strain aging treatment at 250° C. for 5 minutes after the application of a tensile strain of 2.0%, a high tensile strength of 517 MPa or more, a yield ratio of 90% or less, and a uniform elongation of 9% or more were obtained, and a low yield ratio, a high uniform elongation, and superior HIC resistance were achieved.

The structure of each steel material was composed of ferrite, bainite, and martensite-austenite constituent. The area fraction of the martensite-austenite constituent was 0.5 to 5%, and the difference in hardness between the ferrite and bainite was 60 or more in terms of Vickers hardness.

In Nos. 8 to 13, which are Comparative Examples, although the chemical composition was within the scope of the present disclosure, the manufacturing method was outside the scope of the present disclosure. Therefore, any of the structure, the strength, the yield ratio before the strain aging treatment, the yield ratio after the strain aging treatment, and the uniform elongation were insufficient. In Nos. 14 to 18, the chemical composition was outside the scope of the present disclosure, or the manufacturing method was outside the scope of the present disclosure. Therefore, the strength obtained was insufficient, the yield ratio was high, the uniform elongation was low, or cracking occurred in the HIC test. 

The invention claimed is:
 1. A steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance, the steel material having a chemical composition comprising: C: 0.05 to 0.100%, by mass %; Si: 0.01 to 0.50%, by mass %; Mn: 0.5 to 2.5%, by mass %; P: 0.015% or less, by mass %; S: 0.002% or less, by mass %; Cu: 0.20 to 1.00%, by mass %; Mo: 0.01% or less, by mass %; Nb: 0.005 to 0.05%, by mass %; Ti: 0.005 to 0.040%, by mass %; Al: 0.10% or less, by mass %; and N: 0.007% or less, by mass %, with the balance being Fe and inevitable impurities, wherein: the steel material has a metallographic structure including ferrite, bainite, and martensite-austenite constituent, an area fraction of the martensite-austenite constituent is 0.5 to 5.0%, a difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness, and the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less both (i) before a strain aging treatment at a temperature of 300° C. or lower and (ii) after the strain aging treatment.
 2. The steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance according to claim 1, wherein the chemical composition further comprises at least one of: Ni: 0.02 to 0.50%, by mass %; Cr: 1.00% or less, by mass %; V: 0.10% or less, by mass %; Ca: 0.0050% or less, by mass %; and B: 0.0050% or less, by mass %.
 3. A method for manufacturing a steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance, the method comprising: heating steel to a temperature of 1,000 to 1,300° C., the steel having a chemical composition including: C: 0.05 to 0.100%, by mass %, Si: 0.01 to 0.50%, by mass %, Mn: 0.5 to 2.5%, by mass %, P: 0.015% or less, by mass %, S: 0.002% or less, by mass %, Cu: 0.20 to 1.00%, by mass %, Mo: 0.01% or less, by mass %, Nb: 0.005 to 0.05%, by mass %, Ti: 0.005 to 0.040%, by mass %, Al: 0.10% or less, by mass %, and N: 0.007% or less, by mass %, with the balance being Fe and inevitable impurities; subjecting the heated steel to hot rolling at a finish rolling temperature equal to or higher than Ar₃ temperature; then, after hot rolling the steel, subjecting the steel to accelerated cooling from a cooling start temperature of (Ar₃−50) to (Ar₃+30)° C. to a cooling stop temperature of 450 to 650° C. at a cooling rate of 5° C./s or more; and reheating the steel to 550° C. to 750° C. at a heating rate of 0.5° C./s or more immediately after the accelerated cooling; wherein: the steel material has a metallographic structure including ferrite, bainite, and martensite-austenite constituent, an area fraction of the martensite-austenite constituent is 0.5 to 5.0%, a difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness, and the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less both (i) before a strain aging treatment at a temperature of 300° C. or lower and (ii) after the strain aging treatment.
 4. A welded steel pipe produced using the steel material according to claim 1 as a raw material.
 5. A welded steel pipe produced using the steel material according to claim 2 as a raw material.
 6. The method according to claim 3, wherein the chemical composition further comprises at least one of: Ni: 0.02 to 0.50%, by mass %; Cr: 1.00% or less, by mass %; V: 0.10% or less, by mass %; Ca: 0.0050% or less, by mass %; and B: 0.0050% or less, by mass %.
 7. The steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance according to claim 1, wherein the chemical composition does not comprise Mo.
 8. The method according to claim 3, wherein the chemical composition does not comprise Mo. 